The use of photovoltaic devices with systems for concentrating sunlight onto these devices has resulted in the development of methods for optimizing the efficiency and lifetime of devices operating at elevated temperatures while minimizing the losses due to grid design. Concentrator systems often consist of cones, funnels, or lenses used to reflect a wide field of incident light onto a small focal plane where a solar cell is located. These cells are often used in space applications where thermal and radiation damage due to the environment can severly reduce cell lifetime and efficiency. Werthen, J. G., Hamaker, H. C., Virshup, G. F., Lewis, C. R., and Ford, C. W., "High-Efficiency AlGaAs-GaAs Cassegrainian Concentrator Cells," NASA Conferences Publication 2408:61-68 (Apr. 30, 1985); Amano, C., Yamaguchi, M., and Shibukawa, A., "Optimization of Radiation-Resistant GaAs Solar Cell Structures," International PVSEC 1: 845-848 (1987).
The interdiffusion of the material used as an electrical conductor with photo-active semiconductor materials from which current is collected has resulted in the development of diffusion barrier schemes to control the instability of adjacent layers. This interdiffusion, resulting in the breakdown of the structure and loss of efficiency, has plagued existing devices being operated at high temperatures. Many nitrides, borides, and carbides of early transition metals have been suggested for use as diffusion barriers with silicon. M-A. Nicolet and M. Bartur; "Diffusion Barriers in Layered Contact Structures," J. Vac. Sci. Technol., 19: 786 (1981). The use of barriers to prevent the diffusion of a Au overlying contact layer into an ohmic contact metallized layer on GaAs wafers has been discussed in J. Shappirio, et al., J. Vac. Sci. Technol. A, 3: 2255(1985). The survivability of such metallized photovoltaic devices at temperatures up to 700.degree. C. has been explored. These cells showed significant degradation when temperature tested at 700.degree. C., especially when tested in a vacuum. See Horne, W. E., et al., "High Temperature Contact Metallizations for Advanced Solar Cells," Final Report on Contract AFWAL-TR-84-2044, AFWAL/POOC, Wright Patterson Air Force Base, Ohio (Sept. 1981-April 1984).
The grid pattern used in photovoltaic devices contains several loss mechanisms which reduce the available power output. It is desirable to reduce the grid area as the grid blocks out light that would otherwise enter the cell. This factor must be balanced against ohmic and surface recombination losses that are reduced with greater areal coverage by the grid contacts.
The area normally illuminated on a concentrator cell is circular in nature, with the power of the incident light decreasing radially from the center. Thus, the typical grid design for solar cell concentrators has a circular pattern with radial spokes in order to minimize power loss and maximize current collection. See Gregory C. DeSalvo, Ervin H. Mueller and Allen M. Barnett, "N/P GaAs Concentrator Solar Cells with an Improved Grid and Busbar Contact Design," NASA Conference Publication 2408:51-59 (Apr. 30-May 2, 1985); Basore, P. A., "Optimum Grid-Line Patterns for Concentrator Solar Cells Under Nonuniform Illumination," IEEE Photovoltaic Conference Record 84CH2019-8: 637-642 (May 1-4, 1984).
The use of gallium arsenide (GaAs) for solar cells, and of aluminum gallium arsenide (AlGaAs) double-heterostructures in particular, has been disclosed in U.S. Pat. No. 4,547,622 and other references cited therein. Different AlGaAs layers may be used to create a back surface field (BSF) to reflect electrons toward the p-n junction, and/or to reduce unwanted surface recombination.
Even with the many known device improvements, photovoltaic structures continue to suffer efficiency losses due to mechanical and thermal stresses encountered at elevated temperatures. Cells using compound semiconductors in particular, lose their structural and chemical integrity due to decomposition, especially at the higher operating temperatures often encountered with the use of concentrator systems.